This invention relates to scanning apparatus and, more particularly, to an improved holographic spinner system.
There have been previously devised many forms of spinning optical scanners which are utilized for scanning a liquid beam in a repetitive fashion over a surface. A commonly used type employs a multifaceted mirror of pyramidal shape. This type of device has been successively utilized in various applications but has certain drawbacks. For example, the cost of fabricating pyramidal-shaped multifaceted spinners to achieve substantial identity in the reflecting properties of all facets is high, so where relatively high performance characteristics are necessary, one can expect correspondingly great costs. Also, such spinners are often employed at high speed and it is found that the facet surfaces can cause an aerodynamic "drag" which increases the required drive power and, perhaps more seriously, causes inertial deformation which manifests itself as a degradation of system performance.
As an alternative to multifaceted pyramidal mirrors, there has been recently developed a scanning device sometimes known as a "holographic spinner" which employs a plurality of zone-type lenses on a rotating surface, the lenses functioning to focus incident light at a locus of focal points which define a desired scan line. Holographic spinner elements have been fabricated in various shapes with, for example, U.S. Pat. No. 3,614,193 disclosing a spinner having a convex spherical surface, U.S. Pat. No. 3,721,487 disclosing a spinner having a concave spherical surface, and copending U.S. application Ser. No. 514,613 (entitled "LIGHT SCANNING SYSTEM AND METHOD OF MAKING SAME" and assigned to the same assignee as the present invention) disclosing a flat disc spinner surface. The optical symmetries associated with these configurations render them particularly useful in certain applications. For example, it is envisioned that replication techniques, such as the pressing techniques by which phonograph records are made, may ultimately be used to produce low cost holographic spinner elements for use in relatively low cost scanning systems (e.g. in facsimile transmission equipment). This type of replication is facilitated if the zone-type lens is used on a relatively flat spinner surface; e.g., a flat disc spinner or a large radius concave spinner. An operational problem exists, however, when employing a spinner having a flat or a concave spherical shape. Specifically, in these types of systems the principal (or central) ray emerging from the holographic spinner is not normal to the scanning surface or film, the inclination angle generally being greater than 30.degree.. This is evident, for example, in the U.S. Pat. No. 3,721,487 and is necessitated by optical limitations in making the holographic surface on the spinner. (It is well understood by those skilled in the art that as this inclination angle is reduced the "interference" fringes of the holographic pattern move closer together, so very small inclination angles are presently considered unattainable.) As a consequence of the non-normal impinging beam, the reconstructed spot is generally elongated in a cross-scan direction. Also, the position of the reconstructed spot changes significantly with focusing. This position sensitivity of prior art scanners results in a degradation of the scanline pattern since a non-uniformity of the scanning surface is effectively translated to spot movement.
It is one object of the present invention to provide an optical scanning device of the type described, but wherein the principal ray of the light bundle impinging on the scanning surface is constrained to be normal to the scanning surface.